1. Field of the Invention
The present invention relates to semiconductor processing and more particularly to a method of forming a planar surface in a semiconductor device.
2. Description of Related Art
As companies push to miniaturize electronic components, the features which make up the transistor and other silicon based devices must also become smaller. Because these devices are often made from photolithography techniques which require focusing of a mask across an entire semiconductor surface, the planarity of the semiconductor surface has become increasingly important. An uneven surface causes portions of the photomask to be out of focus. An uneven surface also makes it difficult to form subsequent layers of semiconductor structures on top.
A major cause of uneven surfaces is the uneven density of semiconductor features across a surface. One method of eliminating gaps or open areas between semiconductor features which causes uneven subsequent layers is to place dummy structures between the semiconductor features. For purposes of this application, dummy structures are not considered to be semiconductor features. Dummy structure is an artifact which typically is the same material as the semiconductor feature and is formed during the same process with the formation of the semiconductor features but the dummy structure usually do not perform active electrical functions. Traditionally, these dummy structures are inserted into medium and low density open spaces between semiconductor features. The dummy structures are small so that they can fit the various gaps between semiconductor features. An approximately 50% density of artifacts including semiconductor feature and dummy structures to unoccupied area is desired.
Areas of very high density do not need dummy structure because semiconductor features already meet the 50% density criteria. Very low density areas with wide open spaces may also be adequately filled by many small dummy structures. However, medium/low density areas have scattered devices which make it difficult to place adequate numbers of dummy structures. A typical definition of medium/low density is that semiconductor features represent approximately 10-30% of the pattern surface area. Very low density areas are defined to be regions in which the density of semiconductor features is less than the density of semiconductor features in medium/low density regions. Device processes limit how close dummy structures can be placed. These limitations which are based on lithographic and process constraints prevent medium/low density areas populated with several small dummy structures from being adequately filled. As a result, the density of dummy structures in these medium/low density areas between semiconductor features is lower than the density of semiconductor features in other areas of a semiconductor layer.
In general, dummy structures are inserted to achieve a structure density as uniform as possible across a device layer. Structure density or distribution is defined as the density of artifacts on a semiconductor surface. Such artifacts include both dummy structures and semiconductor features. In order to create a more uniform total structure distribution, it is desirable to have different sized dummy structures corresponding to different size open regions. Such a method of forming different size dummy structures is described in the following application.
A second limitation on dummy structure size is that planarization processes cannot handle large open areas completely filled with one oversized dummy structure. Oversized dummy structures which create densities significantly exceeding 50% create an alternative polishing planarization problem. In particular, oversized dummy structures create a raised area in subsequent semiconductor layers. Thus, it is also desirable to adjust dummy structure sizes in large open areas to "break-up" oversized dummy structures.
The following invention will disclose a method and apparatus for creating different size dummy structures for use in semiconductor devices while at the same time preventing the formation of oversized dummy structures.